A polymer having electrolytic groups in its polymer chain has the property of strongly bonding to specific ions and selectively transmitting cations or anions, and it is hence molded in the form of particles, fibers or a membrane and is used in various fields of electrodialysis, diffusion dialysis, a battery diaphragm and the like.
A fuel cell is a cell in which a pair of electrodes are provided on surfaces of a proton-conductive electrolytic membrane, hydrogen gas or methanol as a fuel is supplied to one electrode (fuel electrode) and oxygen gas or air as an oxidizing agent is supplied to the other electrode (air electrode) to produce an electromotive force. Further, water electrolysis refers to the production of hydrogen and oxygen by electrolyzing water with an electrolytic membrane.
Fluorine type electrolytic membranes typified by polyperfluorosulfonic acid membranes known by trade names such as Nafion (registered trademark, supplied by E.I. du Pont de Nemours and Company), Aciplex (registered trademark, supplied by Asahi Kasei Corporation and Flemion (registered trademark, supplied by ASAHI GLASS CO., LTD.), having high proton conductivity, are excellent in chemical stability and they are hence widely used as membranes for fuel cells, water electrolysis and the like.
Further, salt electrolysis refers to the production of sodium chloride, chlorine and hydrogen by electrolyzing a sodium chloride aqueous solution with an electrolytic membrane. In this case, an electrolytic membrane is exposed to chlorine and a high-temperature high-concentration sodium hydroxide aqueous solution, so that a hydrocarbon-containing membrane having poor durability against them cannot be used. As an electrolytic membrane for salt electrolysis, therefore, there is generally used a perfluorosulfonic acid membrane that has durability against chlorine and a high-temperature high-concentration sodium hydroxide aqueous solution and that has a surface into which carboxyl groups are partly introduced for preventing reverse diffusion of ions to be generated.
Meanwhile, fluorine type electrolytes typified by a polyperfluorosulfonic acid membrane have C—F bonds and hence have very high chemical stability. They are therefore used as an electrolytic membrane in/for fuel cells, water electrolysis or salt electrolysis. They are also used as a membrane for the electrolysis of halogenated hydroacid. Further, their proton conductivity is also utilized to widely apply them to humidity sensors, gas sensors and oxygen concentrators.
However, the fluorine type electrolytes have defects that they are difficult to produce and very expensive. The perfluorinated polymer based electrolytic membrane are used only in limited use fields such as a solid polymer type fuel cell for space or military use. And, it has been difficult to apply them to commercial-base use fields such as a solid Polymer Electrolyte Fuel Cell as a low-pollution power source for an automobile and the like.
As a less expensive electrolytic membrane, therefore, there has been proposed an electrolytic membrane formed by sulfonating an aromatic hydrocarbon-based polymer (for example, see Patent Documents 1, 2, 3, 4 and 5) This membrane has an advantage that it can be produced easily and at a low cost as compared with the fluorine type electrolytic membranes typified by Nafion®. However, it has a defect that its oxidation resistance is very low.
For example, Non-Patent Document 1 describes that a sulfonated polyether ether ketone or a polyether sulfone deteriorates at an ether portion adjacent to a sulfonic acid. It is therefore considered that when an electron-donating group is present near a sulfonic acid, oxidation-induced deterioration starts at that point. For improving the oxidation resistance, there has been proposed a sulfonated polyphenylene sulfone whose main chain is composed of only electron-attracting groups and aromatic rings (Patent Document 6). There has been also proposed a sulfonated polysulfone having a sulfonic acid introduced to a portion adjacent to a sulfone group (Non-Patent Document 2).
According to Patent Document 7, however, not only the aromatic hydrocarbon-based polymer electrolytic membrane is deteriorated by oxidation, but also a sulfone group as a proton-conductive substituent bonding directly to an aromatic ring is eliminated under a strong acid at a high-temperature to decrease-the ion conductivity, which is said to be a factor for the deterioration as well. In the sulfonated polyphenylenesulfone or sulfonated polysulfone described in Patent Document 6 or Non-Patent Document 2, the deterioration caused by the elimination of a sulfone group is inevitable. It is therefore undesirable that the proton-conductive substituent is a sulfonic acid, and Patent Document 7 proposes that an alkylsulfonic acid be used in place of the sulfonic acid. This proposal is effective for overcoming a decrease in the ion conductivity caused by the elimination of the sulfonic acid. Since, however, the main chain of an aromatic polymer used contains an electron-donating group, the membrane thereof is poor in oxidation resistance.
Meanwhile, it is expected that an azole polymer, which is excellent in heat resistance and chemical resistance, will make an electrolytic membrane for a fuel cell. As an azole polymer having proton conductivity, for example, there has been proposed a sulfonated azole polymer (Patent Document 8). However, a sulfone group introduced onto an aromatic ring of a polymer as a raw material is liable to undergo a desulfonation reaction due to an acid or heat as described above, and it cannot be said that such a polymer has sufficient durability against use as an electrolytic membrane for a fuel cell. An azole polymer having a hydroxyl group and a process for the production thereof are reported, for example, in Non-Patent Document 3. Further, there is a report of conductivity measurement of the ion implantation product of an azole polymer membrane having a hydroxyl group (Non-Patent Document 4).
In none of these Documents, however, a hydroxyl group has not been regarded as a functional group to conduct ions, and these materials have no sufficient durability against conditions of a fuel cell and use fields.
As a material to be used mainly for fibers, further, there has been proposed a stiff rod polymer based on pyridobisimidazole, which is similar to the above materials (Patent Document 9).                (Patent Document 1) JP-A 6-93114        (Patent Document 2) JP-A 9-245818        (Patent Document 3) JP-A 11-116679        (Patent Document 4) JP-A 11-510198        (Patent Document 5) JP-A 11-515040        (Patent Document 6) JP-A 2000-80166        (Patent Document 7) JP-A 2002-110174        (Patent Document 8) JP-A 2002-146018        (Patent Document 9) WO94/025506        (Non-Patent Document 1) Collected papers of polymers, Vol. 59, No. 8, pages 460-473        (Non-Patent Document 2) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 34, 2421-2438 (1996)        (Non-Patent Document 3) Polymer, 35, (1994) 3091        (Non-Patent Document 4) Polymeric Materials Science and Engineering (1991), 64, 171-2        